1. Field of the Invention
The present invention relates to synthetic silica glass articles and a method for manufacturing them. More particularly, it relates to synthetic silica glass articles, such as, crucibles, containing decreased amounts of metallic impurities and capable of exhibiting high viscosity at high temperatures as high as or even higher than the viscosity of natural quartz glass at such high temperatures. The inventive glass makes excellent melting crucibles for use in a Czochralski-method semiconductor (silicon) single crystal ingot pulling apparatus, or as heat resistant materials used in a diffusion furnace.
2. Description of the Prior Art
Silicon single crystal ingot is manufactured in any of the following methods: the floating zone method (FZ method) wherein a polycrystal silicon ingot obtained by thermally decomposing trichlorosilane is single-crystalized by heating with a high frequency wave; the Czochralski method (CZ method) wherein the same polycrystal silicon is melted in a quartz crucible, a seed crystal is dipped in the melt, and a single crystal is pulled up from the melt; and the magnetic-field applied Czochralski method (MCZ method) which is a special Czochralski method wherein a magnetic field is applied to the melt. Among these methods, the FZ method gives a silicon single crystal of the highest purity since with the other two methods, the quartz crucible holding the silicon melt contains small amounts of impurities, such as, Al and B, which elute into and thereby contaminate the melt. When thus contaminated, the resulting semiconductor material will have a resistance which is lower than expected. The lower portion of the single crystal silicon ingot, which is grown toward the end of the crystallization, tends to have a greater concentration of the impurities. Since the concentration determines the segregation coefficient of the impurities, the resulting single crystal silicon ingot will have nonuniformities in properties in both the axial direction and radial direction, so that the reliability of the product will be poor and quality control becomes difficult.
The contamination of the silicon single crystal occurs not only during the pulling up operation, but also during the diffusion operation. For example, in diffusing boron and phosphorus throughout the silicon wafer, the wafer is heated to about 1000.degree. C. in a furnace. During this procedure, transition metals and alkali metals, such as, iron, copper, sodium, potassium, and lithium eluting from a silica glass furnace tube and other jigs, contaminate the silicon wafer. Consequently, elemental devices incorporating the chips made from the wafer will have increased noise and poor performance properties. Therefore, in order to solve the contamination problem, it is necessary to decrease the concentration of all of the elements regarded as diffusible impurities.
The crucibles used in single crystal pulling apparatus, and especially those used in silicon single crystal pulling apparatuses, are commonly made from natural quartz glass due to its excellent heat-resisting property. However, natural quartz glass contains a relatively large amount of impurities and the resulting contamination of the single crystal ingots is a serious problem. The tendency towards higher integration of semiconductor memories has increased the demand for single crystal semiconductors of higher purity. This has resulted in an increased occurrence of out-of-specification wafers.
Japanese Provisional Patent Publication (Kokai) No. 59-129421 (1984) proposes a method wherein a quartz glass crucible containing no more than 0.5 ppm of alkali metals and no more than 0.03 ppm of copper, and having a viscosity at 1200.degree. C. of not lower than 10.sup.12 poise, is obtained by refining a natural quartz crystal through flotation, steeping the crystal in hydrogen fluoride liquid, melting the crystal for many hours to evaporate alkali metals and copper, and casting it in the shape of a crucible. Further to this, Japanese Provisional Patent Publication (Kokai) No. 60-137892 (1985) proposes a method wherein a quartz glass crucible containing no more than 0.2 ppm of alkali metals and no more than 0.02 ppm of copper, having a viscosity at 1450.degree. C. of 10.sup.10 poise, and an electrical resistance at 1200.degree. C. of not lower than 1.4.times.10.sup.7 .OMEGA..m, is obtained by refining a natural quartz crystal through flotation, arc-melting the crystal, casting it in the shape of a crucible, and conducting a direct current of 10 kV through the crystal under a temperature of 1300.degree. C. in a furnace to thereby move and remove the alkali metals and copper.
Japanese Provisional Patent Publication (Kokai) No. 61-44793 (1986) teaches a quartz glass crucible wherein the inner layer has an OH group content of no less than 200 ppm and a viscosity at 1400.degree. C. of 10.sup.8 -10.sup.9 poise, and the outer layer has an OH group content of no more than 100 ppm and a viscosity at 1400.degree. C. of not lower than 10.sup.10 poise. The quartz glass crucible obtained is capable of allowing the silicon crystal ingot to contain a large number of oxygen atoms which function to adsorb and remove impurities, such as, heavy metals and microdefections existing in the surface areas of wafers sliced from the ingot.
The quartz glass crucibles disclosed by Japanese Provisional Patent Publication (Kokai) No. 59-129421 and Japanese Provisional Patent Publication (Kokai) No. 60-137892 both originate from natural quartz, and to decrease the contents of alkali metals and copper in natural quartz, the methods employ either extended melting periods or electrolysis. However, metallic elements, such as, Al, Ti, Zr, B, and P which exist in considerable amounts in the natural quartz cannot be removed owing to their low diffusion rates. These metallic impurities need be removed since the silicon single crystal contaminated with them will have poor semiconductor properties.
Also, studies have been carried on to develop methods for making a crucible of high purity synthetic silica glass. An example of such methods is to hydrolyze silicon tetrachloride or the like in an oxyhydrogen flame to thereby obtain silica grains. After melting the silica grains, the resulting quartz glass is cast in the shape of a crucible. However, a problem with this method is that since as much as 1000 ppm of OH groups are retained in the glass, the viscosity becomes excessively low at high temperatures, and, in the vacuum atmosphere, the glass foams at high temperatures. Another example of such methods employs a plasma jet flame in place of the oxyhydrogen flame. However a problem is that the cost is high and it is difficult to operate the method on a mass production basis. Also, the so-called sol-gel method is known wherein an alkoxysilane is hydrolyzed in alcoholic solvent to obtain silica, which is then melted to form a synthetic silica glass. However, with this method, although it is possible to obtain high purity material at a relatively low cost, it is disadvantageous because the tendency to retain OH groups is excessive, and because a relatively long time is required for the manufacturing, and also it is not easy to obtain silica glass which maintains a high viscosity at high temperatures.